1. Field of tile Invention
The present invention generally relates to deposition of films onto a substrate. More particularly, the present invention relates to deposition of diffusion barriers and fluorinated silicon glass.
2. Background of the Related Art
As feature sizes have become smaller and multilevel metallization commonplace in, integrated circuits, low dielectric constant films have become increasingly important. Low dielectric constant films are particularly desirable for intermetal dielectric (IMD) layers to reduce the RC time delay of the interconnect metallization being covered to prevent crosstalk between the different levels of metallization, and to reduce device power consumption.
Many approaches to lower dielectric constants have been proposed. One of the more promising solutions is the incorporation of a halogen element, such as fluorine, chlorine or bromine into a silicon oxide layer. Fluorine, the preferred halogen dopant for silicon oxide, lowers the dielectric constant of the silicon oxide film because fluorine is an electronegative atom that decreases the solubility of the overall silicon oxide film. Fluorine-doped silicon oxide films are referred to as fluorosilicate glass films or FSG for short.
In addition to decreasing the dielectric constant, incorporating fluorine in silicon oxide layers also helps to solve common problems encountered in fabricating smaller geometry devices, such as filling closely spaced gaps between metal or polysilicon lines deposited over semiconductor structures. It is believed that because fluorine is an etching species, the introduction of fluorine during deposition of a silicon oxide film introduces an etching effect on the growing film. The simultaneous deposition/etching effect allows FSG films to have improved gap-filling capabilities such that the films are able to adequately cover adjacent metal layers having an aspect ratio of 1.8 or more. Thus, manufacturers desire to include fluorine in various dielectric layers and particularly in intermetal dielectric layers in multilevel structures.
Current integrated circuits generally include various formations of multilevel metal structures that form a high-conductivity, thin-film network fabricated above the silicon surface to connect various active devices through specific electrical paths. During the formation of metal-to-metal and metal-to-silicon contact structures in this thin-film network, openings are etched in the intermetal dielectric layer, such as the doped silicon dioxide film, that separates the substrate or underlying conductive thin film from the overlying conductive thin film. A conductive material, such as copper, aluminum or another metal, is then used to fill the opening and make a connection to the silicon substrate or underlying conductive thin film. Ideally, the impedance to current flow between the silicon and overlying connecting metal layer or between the underlying and overlying connecting metal layers should be as low as possible.
Diffusion barriers play a prominent role in the formation of multilevel metal structures which are present in many integrated circuits. Diffusion of materials between adjacent layers in semiconductor devices is a particular concern to those in the semiconductor industry. Such diffusion or intermixing may be prevented by sandwiching another material or stack of materials between the layers. The role of this third material or stack of materials is to prevent or retard the diffusion of the two materials into each other and hence the layer is often referred to as a diffusion barrier.
With the recent progress in sub-quarter-micron copper interconnect technology, tantalum and tantalum nitride have become popular barrier materials in addition to titanium and titanium nitride. Depending on the application, a diffusion barrier layer may comprise a tantalum layer, a tantalum nitride layer, a tantalum/tantalum nitride stack or other combinations of diffusion barrier materials. The diffusion barrier layer is commonly deposited over the doped silicon oxide film after openings for interconnect structures (contacts or vias) have been etched in the doped silicon oxide film. A metal, such as copper, is then deposited over the diffusion barrier to fill the interconnect feature.
During substrate processing, heat treatment steps in which a substrate is heated to a specified temperature for a specified time are employed for various reasons. For example, an anneal step may be used to repair damage to a substrate after a plasma processing step.
However, when a FSG film is subjected to a temperature greater than about 350xc2x0 C., loosely-bonded (dangling bonds) fluorine atoms and residual fluorine atoms tend to be released from the FSG film. The released fluorine atoms from the FSG film react with the tantalum component of the tantalum nitride barrier layer and form volatile TaF2. TaF2 formation increases the resistance of the interconnect structure and causes significant losses in the adhesion properties between the tantalum nitride layer and the FSG film. The loss in adhesion properties causes the tantalum nitride barrier layer to peel off during subsequent processing of the substrate, resulting in the formation of defects. Similarly, for a titanium based barrier layer, the released fluorine atoms react with the titanium to form TiF, which leads to defect formations on the substrate as TiF2.
From the discussion above, it can be seen that low dielectric constant films, such as FSG and other halogen-doped silicon oxides, are desirable to use as intermetal dielectric layers in multilevel metal structures. However, there is a need to prevent reactions between the halogen-doped silicon oxides and the adjacent diffusion barrier material.
U.S. Pat. No. 5,763,010, by Guo et al, hereby incorporated by reference, illustrates an attempt to stabilize a halogen-doped silicon oxide film and to reduce halogen atoms migration and reaction with adjacent films during subsequent processing. The deposited halogen-doped silicon oxide film is subjected to a degassing step in which the film is briefly heated to a temperature of between about 300xc2x0 C. and about 500xc2x0 C. for between about 35 seconds and about 50 seconds before deposition of the barrier layer. The heat degassing treatment removes loosely bonded halogen atoms. However, the heat degassing treatment may produce more loosely bonded halogen atoms in the halogen-doped silicon oxide film when the substrate has been heated for a longer period of time than the optimal heat degassing treatment time. Furthermore, when the substrate has been heated for a shorter period of time than the optimal heat degassing treatment time, the heat degassing treatment may remove an insufficient amount of loosely bonded halogen atoms in the halogen-doped silicon oxide film. Also, it is generally preferred to minimize the substrate""s exposure to a heated environment.
Therefore, there remains a need for a method to stabilize a halogen-doped silicon oxide film and to prevent loosely bonded halogen atoms from reacting with components of the barrier layer during subsequent processing of the substrate without subjecting the substrate to a heated environment. It would be desirable for the method to improve the adhesion strength between the halogen-doped silicon oxide film and the barrier layer. It would be further desirable for the method to be practiced in an integrated process sequence with other substrate processing such that the method can be practiced in a variety of processing chambers, including both physical vapor deposition chambers as well as chemical vapor deposition chambers.
The present invention generally provides a method for stabilizing a halogen-doped silicon oxide film and preventing loosely bonded halogen atoms from reacting with components of the barrier layer during subsequent processing of the substrate. The invention provides a hydrogen plasma treatment of the halogen-doped silicon oxide film without subjecting the substrate to a heated environment that may damage the substrate and the structures formed on the substrate. The invention also improves the adhesion strength between the halogen-doped silicon oxide film and the barrier layer. Furthermore, the hydrogen plasma treatment can be practiced in a variety of processing chambers of an integrated process sequence, including pre-clean chambers, physical vapor deposition chambers, chemical vapor deposition chambers, etch chambers and other plasma processing, chambers.
In one aspect, the invention provides a method for treating a halogen-doped silicon oxide film, particularly a fluorinated silicon oxide film, deposited on a substrate, comprising exposing the halogen-doped silicon oxide film to a hydrogen plasma. Preferably, the hydrogen plasma treatment is carried out in a pre-clean chamber, and the substrate is transferred without breaking vacuum to another chamber used for depositing the diffusion barrier film. A diffusion barrier film, such as tantalum nitride, is then deposited over the treated halogen-doped silicon oxide film. The hydrogen plasma reactive cleaning process removes the loosely bonded halogen atoms from the silicon oxide film and provides a stable structure for the remaining halogen-doped silicon oxide film. The resulting film does not react with the barrier material and retains its adhesive properties with the barrier film.
Another aspect of the present invention provides a method for treating a fluorinated silicon oxide film deposited on a substrate, comprising reactive cleaning the fluorinated silicon oxide film using a hydrogen plasma. Preferably, the hydrogen plasma comprises a plasma of a processing gas comprising hydrogen and a carrier gas, wherein the processing gas contains between about 5% and about 50% hydrogen. The fluorinated silicon oxide film is subjected to the hydrogen plasma treatment for between about 10 seconds and about 300 seconds to remove the loosely bonded fluorine atoms in the fluorinated silicon oxide film and to stabilize the fluorinated silicon oxide film. The hydrogen plasma treatment improves adhesion of a subsequently deposited barrier film, such as a tantalum nitride film, because there are no loosely bonded fluorine atoms to be released during subsequent high temperature processes to form a refractory fluoride compound between the films.